DESCRIPTION

Tc
is used to configure Traffic Control in the Linux kernel. Traffic Control consists
of the following:

SHAPING

When traffic is shaped, its rate of transmission is under control. Shaping may
be more than lowering the available bandwidth - it is also used to smooth out
bursts in traffic for better network behaviour. Shaping occurs on egress.

SCHEDULING

By scheduling the transmission of packets it is possible to improve interactivity
for traffic that needs it while still guaranteeing bandwidth to bulk transfers. Reordering
is also called prioritizing, and happens only on egress.

Traffic exceeding a set bandwidth may also be dropped forthwith, both on
ingress and on egress.

Processing of traffic is controlled by three kinds of objects: qdiscs,
classes and filters.

QDISCS

qdisc
is short for 'queueing discipline' and it is elementary to
understanding traffic control. Whenever the kernel needs to send a
packet to an interface, it is
enqueued
to the qdisc configured for that interface. Immediately afterwards, the kernel
tries to get as many packets as possible from the qdisc, for giving them
to the network adaptor driver.

A simple QDISC is the 'pfifo' one, which does no processing at all and is a pure
First In, First Out queue. It does however store traffic when the network interface
can't handle it momentarily.

CLASSES

Some qdiscs can contain classes, which contain further qdiscs - traffic may
then be enqueued in any of the inner qdiscs, which are within the
classes.
When the kernel tries to dequeue a packet from such a
classful qdisc
it can come from any of the classes. A qdisc may for example prioritize
certain kinds of traffic by trying to dequeue from certain classes
before others.

FILTERS

A
filter
is used by a classful qdisc to determine in which class a packet will
be enqueued. Whenever traffic arrives at a class with subclasses, it needs
to be classified. Various methods may be employed to do so, one of these
are the filters. All filters attached to the class are called, until one of
them returns with a verdict. If no verdict was made, other criteria may be
available. This differs per qdisc.

It is important to notice that filters reside
within
qdiscs - they are not masters of what happens.

CLASSLESS QDISCS

The classless qdiscs are:

[p|b]fifo

Simplest usable qdisc, pure First In, First Out behaviour. Limited in
packets or in bytes.

pfifo_fast

Standard qdisc for 'Advanced Router' enabled kernels. Consists of a three-band
queue which honors Type of Service flags, as well as the priority that may be
assigned to a packet.

red

Random Early Detection simulates physical congestion by randomly dropping
packets when nearing configured bandwidth allocation. Well suited to very
large bandwidth applications.

sfq

Stochastic Fairness Queueing reorders queued traffic so each 'session'
gets to send a packet in turn.

tbf

The Token Bucket Filter is suited for slowing traffic down to a precisely
configured rate. Scales well to large bandwidths.

CONFIGURING CLASSLESS QDISCS

In the absence of classful qdiscs, classless qdiscs can only be attached at
the root of a device. Full syntax:

tc qdisc add dev
DEV
root
QDISC QDISC-PARAMETERS

To remove, issue

tc qdisc del dev
DEV
root

The
pfifo_fast
qdisc is the automatic default in the absence of a configured qdisc.

CLASSFUL QDISCS

The classful qdiscs are:

CBQ

Class Based Queueing implements a rich linksharing hierarchy of classes.
It contains shaping elements as well as prioritizing capabilities. Shaping is
performed using link idle time calculations based on average packet size and
underlying link bandwidth. The latter may be ill-defined for some interfaces.

HTB

The Hierarchy Token Bucket implements a rich linksharing hierarchy of
classes with an emphasis on conforming to existing practices. HTB facilitates
guaranteeing bandwidth to classes, while also allowing specification of upper
limits to inter-class sharing. It contains shaping elements, based on TBF and
can prioritize classes.

PRIO

The PRIO qdisc is a non-shaping container for a configurable number of
classes which are dequeued in order. This allows for easy prioritization
of traffic, where lower classes are only able to send if higher ones have
no packets available. To facilitate configuration, Type Of Service bits are
honored by default.

THEORY OF OPERATION

Classes form a tree, where each class has a single parent.
A class may have multiple children. Some qdiscs allow for runtime addition
of classes (CBQ, HTB) while others (PRIO) are created with a static number of
children.

Qdiscs which allow dynamic addition of classes can have zero or more
subclasses to which traffic may be enqueued.

Furthermore, each class contains a
leaf qdisc
which by default has
pfifo
behaviour though another qdisc can be attached in place. This qdisc may again
contain classes, but each class can have only one leaf qdisc.

When a packet enters a classful qdisc it can be
classified
to one of the classes within. Three criteria are available, although not all
qdiscs will use all three:

tc filters

If tc filters are attached to a class, they are consulted first
for relevant instructions. Filters can match on all fields of a packet header,
as well as on the firewall mark applied by ipchains or iptables.

Type of Service

Some qdiscs have built in rules for classifying packets based on the TOS field.

skb->priority

Userspace programs can encode a class-id in the 'skb->priority' field using
the SO_PRIORITY option.

Each node within the tree can have its own filters but higher level filters
may also point directly to lower classes.

If classification did not succeed, packets are enqueued to the leaf qdisc
attached to that class. Check qdisc specific manpages for details, however.

NAMING

All qdiscs, classes and filters have IDs, which can either be specified
or be automatically assigned.

IDs consist of a major number and a minor number, separated by a colon.

QDISCS

A qdisc, which potentially can have children,
gets assigned a major number, called a 'handle', leaving the minor
number namespace available for classes. The handle is expressed as '10:'.
It is customary to explicitly assign a handle to qdiscs expected to have
children.

CLASSES

Classes residing under a qdisc share their qdisc major number, but each have
a separate minor number called a 'classid' that has no relation to their
parent classes, only to their parent qdisc. The same naming custom as for
qdiscs applies.

FILTERS

Filters have a three part ID, which is only needed when using a hashed
filter hierarchy.

UNITS

All parameters accept a floating point number, possibly followed by a unit.

Bandwidths or rates can be specified in:

kbps

Kilobytes per second

mbps

Megabytes per second

kbit

Kilobits per second

mbit

Megabits per second

bps or a bare number

Bytes per second

Amounts of data can be specified in:

kb or k

Kilobytes

mb or m

Megabytes

mbit

Megabits

kbit

Kilobits

b or a bare number

Bytes.

Lengths of time can be specified in:

s, sec or secs

Whole seconds

ms, msec or msecs

Milliseconds

us, usec, usecs or a bare number

Microseconds.

TC COMMANDS

The following commands are available for qdiscs, classes and filter:

add

Add a qdisc, class or filter to a node. For all entities, a
parent
must be passed, either by passing its ID or by attaching directly to the root of a device.
When creating a qdisc or a filter, it can be named with the
handle
parameter. A class is named with the
classid
parameter.

remove

A qdisc can be removed by specifying its handle, which may also be 'root'. All subclasses and their leaf qdiscs
are automatically deleted, as well as any filters attached to them.

change

Some entities can be modified 'in place'. Shares the syntax of 'add', with the exception
that the handle cannot be changed and neither can the parent. In other words,
change
cannot move a node.

replace

Performs a nearly atomic remove/add on an existing node id. If the node does not exist yet
it is created.

link

Only available for qdiscs and performs a replace where the node
must exist already.

FORMAT

The show command has additional formatting options:

-s, -stats, -statistics

output more statistics about packet usage.

-d, -details

output more detailed information about rates and cell sizes.

-r, -raw

output raw hex values for handles.

-p, -pretty

decode filter offset and mask values to equivalent filter commands based on TCP/IP.